There are applications in the design and manufacture of commercial products in which it is desirable to apply a thermal spray metal coating to a base metal surface. There are different reasons for the application of such a coating. One important reason is that the applied coating may be more wear or corrosion resistant than the base layer.
In recent years, aluminum pistons and aluminum engine blocks have been used in automotive engines, but scuffing and wear due to the motion between the piston and the cylinder wall has created a problem. U.S. Pat. No. 5,080,056, which issued on Jan. 14, 1992, discloses this problem and the efforts made to solve it. U.S. Pat. No. 5,080,056 teaches a method of forming a scuff and wear resistant liner in a relatively low-silicon content aluminum alloy cast engine block. It discloses that engine blocks of a suitable low-silicon aluminum alloy, such as the aluminum 319 alloy, are readily cast into an engine block, and aluminum-bronze alloy compositions are applied by a thermal spray process onto the internal diameter of the cylinder bores of the aluminum casting. The patent discloses that before the thermal-sprayed composition is applied to the cylinder bore, it has to be machined to a suitable oversize dimension and then thoroughly cleaned and degreased so as to be in suitable condition for the thermal-sprayed coating to be adhered to the walls of the cylinder bore.
In the thermal spray technique, a high velocity oxy-hydrocarbon fuel practice is employed to melt and atomize an aluminum-bronze composition. The atomized droplets are sprayed onto the cylinder wall portions of the casting to form a dense coating of suitable thickness. There are, of course, many other applications in which it is desired to apply a thermal spray coating on a metal surface. However, this example of the engine block illustrates the problems and practices that arise in the formation of durable and adherent coatings by this technique.
Thermal spray methods differ in the way that the coating alloy is melted and atomized and propelled against a surface to be coated. For example, melting may be accomplished by electrical means, by plasma heating or by heating with hot combustion gases. A suitable hot gas is typically used to atomize and propel the molten metal against the target surface. The droplets solidify on the colder surface and fuse to form a dense coating.
In any event, in the application of thermal spray coatings, regardless of the particular technique, it has been a common practice to clean, roughen or abrade by blasting a grit such as small ground pieces of glass, aluminum oxide, silicon carbide, etc., that would roughen the surface, and then reclean the surface before the thermal spray coating is to be applied. For example, in the cast aluminum engine block application described above, the cylinder bore portions of the casting would be bored or otherwise machined slightly oversized to accommodate the thermal spray coating. Following this machining operation, it is necessary to solvent clean or degrease the cylinder bore portion of the casting so as to remove machine chips, lubricants and other dirt. Following the solvent cleaning operation, the surface of the cylinder bore is roughened by blasting with a commercial grit material, e.g., aluminum oxide, glass, silicon carbide or chilled iron of -30/+80 mesh size. Grit blasting roughens the surface so as to provide increased surface for adhesion and mechanical bonding between the base metal and the thermal spray coating. However, grit blasting creates the problem of ensuring that all of the grit is removed from the engine block in order to avoid the grit or abrasive contaminating parts of the engine. Further, the grit itself could probably lodge in crevices of the engine block or the cylinder bore surface itself. Thus, the use of grit or abrasives to roughen the surface requires subsequent cleaning of the entire area where the grit may be, which is a time-consuming operation. Also, there is no assurance that all of the grit has been washed out completely. In fact, it is practically impossible to assure all the grit has been removed from the areas where it may be contained as a result of the blasting operation.
Another disadvantage of using grit or abrasive slurries is that the abrasive or grit can even contaminate the surface being treated and although in many instances the roughed surface is suitable to hold the coating, increased tenacity of the roughed surface is desired. In accordance with the preferred embodiments of our invention, these problems are eliminated in a more economical way than one skilled in the art would ever conceive.
Accordingly, it is an object of the present invention to provide a simple, more efficient method of cleaning and toughening the surface of a metal alloy, especially the cast aluminum alloy of an engine's cylinder bore so that it is receptive to a thermal spray applied coating. It is a more specific object of the present invention to provide a one-step method of cleaning and roughening the machined aluminum alloy surface of an engine's cylinder bore so as to provide a grit-free engine and a grit-free improved surface texture for an adherent thermal spray applied metal coating.